In an optical system of a micro structure produced using a micro electro mechanical system (MEMS) technology, an optical switch is used, which directly switches an optical signal without converting the optical signal into an electric signal. Such an optical switch is advantageous in reducing a number of parts or achieving a high-speed switching operation. Especially, a multi-channel micro mirror array fabricated using a micro machining technique is compact and able to achieve high-integration. Thus, a development of such a micro mirror array has been progressed.
It is known that an actuator formed by vertical comb-shaped electrodes can provide a large swing angle (deflection angle) at a low voltage in a drive part of a micro mirror device. FIGS. 1A, 1B and 1C illustrate a micro mirror device having an actuator formed by vertical comb-shaped electrodes. FIG. 1A is a plan view of the micro mirror device. FIG. 1B is a cross-sectional view taken along a line I-I of FIG. 1A. FIG. 1C is a cross-sectional view taken along a line II-II of FIG. 1A.
The micro mirror device illustrated in FIGS. 1A, 1B and 1C has a movable part 1 having a surface on which a mirror 1a reflecting light is formed. The movable part 1 is swingably supported by a pair of torsion bars 2 on an inner frame 3. The inner frame 3 is swingably supported by a pair of torsion bars 4 on an outer frame 5.
A vertical comb-shaped electrode actuator 6 is formed in a portion where the movable part 1 and the inner frame 3 face each other. The movable part 1 can be caused to incline to the inner frame 3 by actuating the vertical comb-shaped electrode actuator 6. The torsion bars 2 are formed as beams having a small width so as to be subjected to a torsional deformation.
Also a vertical comb-shaped electrode actuator 7 is formed in a portion where the inner frame 3 and the outer frame 5 face each other. The inner frame 3 can be caused to incline to the outer frame 5 by actuating the vertical comb-shaped electrode actuator 7. The torsion bars 4 are formed as beams having a small width so as to be subjected to a torsional deformation.
The mirror 1a of the movable part 1 can be inclined in an arbitrary direction by causing the movable part 1 and the inner frame 3 to incline as mentioned above. Thereby, a light incident on the mirror 1a can be reflected in an arbitrary direction in 360 degrees.
The above-mentioned micro mirror device can be fabricated using an SOI (silicon-on-insulator) substrate. The SOI substrate is formed by an SiO2 layer having a thickness of several micrometers being sandwiched between Si layers having several hundreds micrometers. The mirror 1a is provided by forming, for example, an Au layer having a high reflectance on the surface of the movable part 1.
Normally, the micro mirror device is fixed to a housing or a package (PKG) of an optical device via a wiring board, or fixed directly to the housing (PKG). FIG. 2 is a cross-sectional view of a micro mirror device fixed to a housing (PKG) through a wiring board.
In FIG. 2, the outer frame 5 of the micro mirror device is fixed to the wiring board 11 by a joining material 10. As the joining material 10, if an electric conductivity is required between the outer frame 5 and the wiring board 11, an Au ball bump, a solder, an Ag paste or the like is used. If an electric conductivity is not required between the outer frame 5 and the wiring board 11, an adhesive such as a resin may be used as the joining material 10. Normally, a portion of the outer frame 5 where the outer frame 5 is fixed by the joining material 10 is located at four corners of the outer frame 5 having a square shape as illustrated in FIG. 1A, or in the vicinity of two outer sides facing the outer frame 5.
The wiring board 11 to which the micro mirror device is fixed is fixed to the housing (PKG) 13 by a joining material 12. Generally, the housing (PKG) 13 is formed of a metal such as a stainless steel.
In the fixing structure of the micro mirror device as illustrated in FIG. 2, there is a possibility of a stress being applied to the outer frame 5, which results in a slight deformation of the outer frame 5. For example, if the joining material 10 contracts when the joining material 10 is cured, a stress is generated in the outer frame 5. Or, if an ambient temperature of the micro mirror device changes, a stress is generated in the outer frame 5 due to a difference between a coefficient of thermal expansion of the micro mirror device and a coefficient of thermal expansion of the wiring board 11 and the housing (PKG). Especially, because the width of the torsion bars 4 extending between the outer frame 5 and the inner frame 3 is small, the torsion bars 4 easily deform if a stress is generated in the outer frame 5.
If the torsion bars 4 deform, a resonance frequency in the torsion vibration system of a portion containing the torsion bars 4 is changed, which results in a problem in that the characteristics of the micro mirror device is undesirably changed. This problem occurs in the torsion bars 4 connecting the inner frame 3 and the outer frame 5, which is fixed by the joining material 10, but the problem rarely occurs in the torsion bars 2 connecting the movable part 1 and the inner frame 3, which is not fixed by a joining material.
As mentioned above, in a micro electro mechanical device such as a micro mirror device, a stress is generated in the device due to a fixing structure, and there is a problem in that the characteristics of the device is changed due to the stress.
Thus, in a gyroscope sensor using the MEMS technology, it is suggested to reduce an internal stress in a Y direction (width direction) in a support board by providing a stress relaxation groove (for example, refer to Patent Document 1). Additionally, in a micro optical part formed using a micro machining technique, when a structure to cause a micro mirror to reciprocally vibrate using a torsion beam as a rotational axis is adopted, it is suggested to form a separation groove in a mirror substrate (for example, refer to Patent Document 2). Further, it is suggested to suppress a fluctuation of a resonance frequency of a vibration board by providing a slit-like etched hole in a vibration electrode board to release an internal stress generated in the vibration electrode board (refer to Patent Document 3).    Patent Document 1: Japanese Laid-Open Patent Application 2005-292117    Patent Document 2: Japanese Laid-Open Patent Application 2004-37886    Patent Document 3: Japanese Laid-Open Patent Application 2006-157863
In a micro electro mechanical device such as the above-mentioned micro mirror device, there are various structures of a drive portion or a fixing portion. Accordingly, it is desirous to use a method which can optimally relax a stress in consideration of a fixing method and a fixing position and a positional relationship with a drive portion.
Thus, it is desirous to develop a micro electro mechanical device having a structure in which a stress generated in a fixing portion is hardly transmitted to other portions such as a beam supporting a movable portion.